Variable fiber-optic attenuators (VOA) are the basic building blocks for several key optical systems. Presently, these attenuators are required as equalizers in wavelength division multiplexed (WDM) optical communication systems using non-uniform gain optical amplifiers. Other important applications include polarization dependent loss compensation in fiber optic networks, optical component testing, and optical receiver protection. Hence, a variable fiber-optic attenuator with fast sub-microseconds duration speed with exceptionally high attenuation dynamic range (e.g., 50 dB) control is a present challenge to the optical community.
So far, different types of attenuators have been realized but all lack the microsecond speed and high (50 dB or more) dynamic range in a compact high laser damage threshold package. Sliding block mechanical attenuators have excellent optical characteristics but they adjust very slowly, e.g., one-half to one second. Waveguide thermo-optic attenuators have better response time (e.g., in milliseconds) but they do not offer the ultra-high dynamic range as in T. V. Clapp, S. Day, S. Ojha and R. G. Peall, “Broadband variable optical attenuator in silica waveguide technology,” ECOC '98, pp. 301-302, September 1998; and S. -S. Lee, Y.-S. Jin, Y.-S. Son, and T.-K. Yoo, “Polymeric tunable optical attenuator with an optical monitoring tap for WDM transmission network,” IEEE Photon. Technol. Lett. 11, pp. 590-592, 1999. Liquid crystal-based attenuators also suffer from the slow speeds much like thermo-optics. Micromirror-based reflective optical attenuators offer good dynamic range and better response time as in C. R. Giles, V. Aksyuk, B. Barber, R. Ruel, L. Stulz, and D. Bishop, “A silicon MEMS optical switch attenuator and its use in lightwave subsystems,” IEEE J Select. Top. Quant.Elect. 5, pp. 18-25, 1999, but they do not match the exceedingly fast switching time offered by acousto-optics.
Over the years, attempts have been made to realize AO FO VOAs as AO technology has speeds in the sub-microseconds regime. These include an in-line fiber attenuator in K. Jedrzejewski, M. Franczyk, and A. Leszczynski, “Acousto-optically tuned single-mode in-line fiber attenuator,” Proc. SPIE 3731, pp. 103-106, 1999, where the maximum tuning range achieved was limited to 13 dB. In addition, both bulk and fiber-based AO tunable filters (or AOTFs) have been used for optical gain controls, particularly for simultaneous multiple wavelengths. AO based attenuators are also available from commercial vendors today such as Intraction Corp., Bellwood, Ill. and Brimrose Corp., Baltimore, Md. Prior art references where AO devices have been used as wavelength selective switches and optical power level controllers are listed as: K. W. Cheung, M. M. Choy, and H. Kobrinski, “Electronic Wavelength Tuning Using Acoustooptic Tunable Filter with Broad Continuous Tuning Range and Narrow Channel Spacing,” IEEE Photonics Technology Letters, Vol. 1, pp.38-40, No. 2, February 1989; D. A. Smith, J. E. Baran, J. J. Johnson, and K. W. Cheung, “Integrated-optic acoustically tunable filters for WDM networks,” IEEE J. Select. Areas Commun., Vol. 8, pp. 1151-1159, 1990; J. L. Jackel, J. E. Baran, G. -K Chang, M. Z. Iqbal, G. H. Song, W. J. Tomlinson, D. Fritz, and R. Ade, “Multichannel Operation of AOTF Switches: Reducing Channel-to-Channel Interaction,” IEEE Photonics Technology Letters, Vol. 7, pp. 370-372, No. 4, April 1995; F. Tian and H. Herrmann, “Interchannel Interference in Multiwavelength Operation of Integrated Acousto-Optical Filters and Switches,” Journal of Lightwave Technology, Vol. 13, pp. 1146-1154, No. 6, June 1995; D. A. Smith, et.al., “Evolution of the acousto-optic wavelength routing switch,” IEEE/OSA Journal of Lightwave Tech, Vol.14, No.6, June 1996; N. A. Riza, “Wavelength Switched Fiber-Optically Controlled Ultrasonic Intracavity Probes,” IEEE LEOS Ann. Mtg. Digest, pp.31-36, Boston, 1996; N. A. Riza, “Photonically Controlled Ultrasonic Arrays: Scenarios And Systems,” IEEE International Ultrasonics Symp. Digest, pp.1545-1550, San Antonio, 1996; N. A. Riza, “Acousto-Optic Device-based High Speed High Isolation Photonic Switching Fabric for Signal Processing,” Optics Letters, Vol.22, Jul. 1, 1997; D. Ostling and H. E. Engan, “Narrow-band acousto-optic tunable filtering in a two-mode fiber,” Optics Letters, Vol.20, pp. 1247-1249, 1995; H. S. Kim, S. H. Yun, I. K. Kwang, and B. Y. Kim, “All-fiber acousto-optic tunable filter notch filter with electronically controlable spectral profile,” Optics Letters, Vol.22, pp. 1476-1478, 1997; T. A. Birsk, P. St. J. Russell, and C. N. Pannell, “Low power acousto-optic device based on a tapered single-mode fiber,” IEEE Photonics Technology Letters, Vol. 6, pp. 725-727, 1994; S. G. Farwell, M. N. Zervas, and R. L Laming, “2×2 fused fiber null couplers with asymmetric wasit cross sections for polarization independent (<0.01 dB) switching,” Journal of Lightwave Technology, Vol. 16, pp. 1671-1679, 1998; S. H. Huang, X. Y. Zou, S.-M. Hwang, A. E. Wilner, Z. Bao, and D. A. Smith, “Experimental demonstration of dynamic network equalization of three 2.5 Gb/s WDM channels over 100 km using acousto-optic tunable filters,” IEEE Photonics Technology Letters, Vol. 8, pp. 1243-1245, 1996; H. S. Kim, S. H. Yun, H. K. Kim, N. Park, and B. Y. Kim, “Actively gain-flattened erbium-dopped fiber amplifier over 35 nm by using all fiber acousto-optic tunable filters,” IEEE Photonics Technology Letters, Vol. 10, pp. 790-792, 1998; J.-X. Cai, K.-M. Feng, X. Chen, A. E. Willner, D. A. Smith, C.-H. Lee, and Y.-J. Chen, “Experimental demonstration of dynamic high-speed equalization of three WDM channels using acousto-optic modulators and a wavelength demultiplexer,” IEEE Photonics Technology Letters, Vol. 9, pp. 678-680, 1997; T. Nakazawa, M. Doi, S. Taniguchi, Y. Takasu, and M. Seino, “Ti:LiNbO3 AOTF for 0.8 nm channel-spaced WDM,” in Post-Deadline papers OFC'98, 1998, PDI; N. A Riza and J. Chen, “Ultrahigh 47-dBoptical drop rejection multiwavelength adddropfilter using spatial filteringand dual bulk acousto-optic tunable filters,” Optics Letters, Vol.23, pp. 945-947, 1998; E. R Mueller, R. A. Hart, W. A. Veronasi, and F. T. Olender, “System o control the power of a beam,” U.S. Pat. No. 6,089,076, Jul. 18, 2000; S. Scmid and P. C. Sistemi, “Acoustooptic devices having an acoustic attenuator structure,” U.S. Pat. No. 6,195,476, Feb. 27, 2001; F. Tian and H. Hermann, “Interchannel interference in multiwavelength operation of integrated acousto-optical filters and switches,” Journal of Lightwave Technology, Vol. 13, pp. 1146-1154, 1998; T. E. Dirnmick, G. Kakarantzas, T. A. Birsk, A. Diez,a and P. St. J. Russell, “Compact all-fiber acousto-optic tunable filters with bandwidth-length product,” IEEE Photonics Technology Letters, Vol. 12, pp. 1210-1212, 2000; K. W. Cheung, “Acousto-optic tunable filters in narrowband WDM networks: System issues and network applications,” IEEE Journal of Selected Areas in Communication, Vol. SAC-8, pp. 1015-1024, 1990; K.-W. Chueng, “Switch for selectively switching optical wavelengths,” U.S. Pat. No. 4,906,064, March 1990; H. Hermann, A. Modlich, Th. Muller, and W. Sohler, “Double-stage, integrated, acousto-optical add drop multiplexers with improved crosstalk performance,” Proc. ECOC'97 (Edinburgh 1997), Vol. 3, pp. 10-13, 1997; D. A. Smith, R. S. Chakravarthy, Z. Bao, J. E. Baran, J. L. Jackel, A. d'Alessandro, D. J. Fritz, S. H. Huang, X. Y. Zou, S.-M. Hwang, A. E. Willner, and K. D. Li, “Evolution of the acousto-optic wavelength routing switch,” Journal of Lightwave Technology, Vol. 14, pp. 1005-1018, 1996; H. Hermann, K. Scaffer, andSh. Schmidt, “Low-loss tunable integrated acousto-optical wavelength filter in LiNbO3 with strong sidelobe supression,” IEEE Photonics Technology Letters, Vol. 10, pp. 120-122, 1997; H. Hermann, A. Modlich, Th. Muller, W. Sohler, and F. Wehrmann, “Advanced integrated, acousto-optical switches, add-drop multiplexers and WDM cross-connects in LiNbO3,” Proc. ECIO'97 (Stockholm 1997), April 1997; J. Sharony, K.-W. Chueng, and T. E. Stern, “The wavelength dilation concept in lightwave networks-Implementation and system considerations,” Journal of Lightwave Technology, Vol. 11, pp. 900-907, 1993; G. H. Song, “Asymmetric dilation of multiwavelength cross-connect switches for low-crosstalk WDM optical networks,” Journal of Lightwave Technology, Vol. 15, pp. 430-436, 1997. A major problem with past AO device-based attenuators is two fold. First, when these prior art VOAs employ the DC or undiffracted beam from the AO device as the output beam of the VOA, the dynamic range is severely limited to say 10 dB. This is because AO devices work on the principle of Bragg volume diffraction and even a high 90% diffraction with typically a watt of radio frequency (RF) AO device drive power leads to a 10% through light, implying a near 10:1 power reduction or 10 dB attenuation value for the VOA. To solve this DC as output beam problem, the diffracted first order beam can be used as the VOA output beam. In this case, the VOA dynamic range is greatly enhanced because a zero RF drive signal to the AO device leads to essentially no light at the VOA output port/Once an RF is applied to the AO device, the input light gets diffracted and deflected to the output port resulting in lower VOA attenuation. The key limitation of this way to increase VOA dynamic range is that the diffracted beam in an AO device always undergoes a Doppler frequency shift equal to the RF, implying that this VOA has a permanent frequency shift on the input optical carrier. This doppler problem can negatively impact WDM systems, where wavelengths are being tightly allocated and maintained.
Recently N. A. Riza in U.S. Pat. No. 6,282,336 for “High Speed Fiber-Optic Switch,” issued Aug. 28, 2001 and in N. A. Riza and Z. Yaqoob, “High-speed programmable optical attenuator,” Proceedings of SPIE, Vol. 4046, paper 10, Orlando, Fla., Apr. 26, 2000 and N. A. Riza and Z. Yaqoob, “Sub-microsecond speed variable optical attenuator using acousto-optics,” IEEE Photonic Technology Letters, Vol.13, pp.693-695, July 2001., proposed a new technique to simultaneously solve both the doppler and dynamic range problem of the AO-based VOA. The technique proposed employed the concept of double diffraction and frequency driven output beam misalignment. FIG. 1. shows an embodiment of this concept where the two AO devices are arranged to generate opposing Doppler shifts on the diffracted beams leading to a VOA with both high dynamic range and no wavelength shifts on the input beam.
The purpose of this invention as partly described earlier in N. A. Riza, R Akbar, S. Sumriddetchkajorn, F. Perez, and M. J. Mughal, “47 dB dynamic range sub-microsecond switching speed variable fiber-optic attenuator for fast transient fiber-optic,” OSA Topical Meeting on Photonics Switching, Postdeadline Paper PDP2, June 15, Monterey, Calif., 2001, and M. J. Mughal and N. A. Riza, “65 dB dynamic range 2.8 microseconds switching speed variable fiber-optic attenuator,” 27th European Conference on Optical Communications (ECOC) Proceedings, Vol.6, Postdeadline Papers, PD.A.1.6, Oct. 4, Amsterdam, Netherlands, 2001, and M. J. Mughal and N. A. Riza, “Compact acousto-optic high speed variable attenuator for high power applications,” IEEE Photonic Technology Letters, April 2002, is to show how the double diffraction concept used in the prior patent application cited in the earlier paragraph can be extended to multi-diffraction (i.e., two or more diffractions) structures to realize more efficient and compact Bragg diffraction-based VOA modules.